SUMMARY Macrophages and neutrophils play major roles in host defense against microbial infections. In order to perform this function, these cell types must ingest and destroy pathogens, generally in phagosomes, as well as secrete a number of products that signal other immune cells to respond. Generation of low organellar pH is primarily driven by the V-ATPases, proton pumps that use cytoplasmic ATP to load H+ into the organelle. Alongside the pumps are various channels that shunt the transmembrane potential generated by movement of protons;in different organelles these comprise H+ channels, K+ channels and Cl- channels. Nevertheless, the contribution of these pathways to maintenance of intraorganellar pH is poorly studied. Recently, we demonstrated that murine alveolar macrophages (AMs) but not neutrophils employ the CFTR (Cystic Fibrosis Transmembrane conductance Regulator) Cl? channel as a major shunt mechanism. Lysosomes and phagosomes in murine cftr/- AMs failed to acidify and the cells were deficient in bacterial killing compared to wild-type controls. We have also shown that AMs lacking CFTR are deficient in stimulus-induced secretion. Here we propose to extend these observations by investigating the role of Cl- flux in a common set of phagocyte core functions namely, organellar acidification, granule secretion, and microbicidal activity. We will compare the phagocytic and secretory activities of alveolar and peritoneal macrophages as well as neutrophils and the anion channels that are involved in the regulation of these activities. CFTR and ClC-3 chloride channels are the most reasonable channels with which to begin our studies given our preliminary data on CFTR in murine AMs and that of others in human and mouse neutrophils showing a differential functional dependence on either ClC-3 or CFTR depending upon the species. Utilizing primary cells obtained from normal, Cftr-deficient (or mutant) and ClC3-deficient mice as well as human cells from non-CF and CF patients, we will use a variety of molecular, immunochemical, microscopic and electrophysiological techniques, well-established in our laboratories, to provide a multi-faceted systems approach to the problem. NARRATIVE Patients with CF are highly susceptible to chronic bacterial infection. To date, lung dysfunction in CF has been largely attributed to depletion of the liquid layer covering the upper airway epithelium with a consequent accumulation of mucus that is thought to contribute to the persistence of bacteria in the airway tree. We propose that an additional defect may be attributable to a failure of CFTR-deficient alveolar macrophages to exhibit vigorous bactericidal activity. Defects in the behavior of the innate immune system could have important consequences for microbial defense in CF patients.